A static-pressure bearing apparatus has been conventionally used as an apparatus such as an apparatus for processing, an apparatus for inspection, and an apparatus for manufacturing a semiconductor, all of which require highly accurate positioning. An example of a conventional static-pressure bearing apparatus is shown in FIG. 9. As shown in FIG. 9 (A), the static-pressure bearing apparatus 100 consists of the fixed body 120 and the moving body 110. The fixed body 120 is formed as a rectangle. The moving body 110 is formed to surround the fixed body 120. Also, the moving body 110 is configured to be movable along the fixed body 120. An enlarged illustration of the cross-section X-X of FIG. 9 (A) is shown in FIG. 9 (B). The static-pressure pad 111 is formed on the sliding surface 110a of the moving body 110. The static-pressure pad 111 comprises the outlet 111a and the vent groove 111b. The outlet 111a communicates with a source of pressurized gas and discharges the pressurized gas. The vent groove 111b distributes the pressurized gas, which is discharged from the outlet 111a, along the sliding surface 110a. For example, the vent groove 111b is formed to have a rectangular cross-section. The groove is about 30 microns deep and 1,000 microns wide.
The static-pressure bearing apparatus 100 may float the moving body 110 above the fixed body 120 by discharging pressurized gas from the static-pressure pad 111 into the bearing clearance G1 that is formed between the sliding surface 110a of the moving body 110 and the fixed body 120. Also, the static-pressure bearing apparatus 100 may move the moving body 110 along the direction that the fixed body 120 is extended. In this way, the static-pressure bearing apparatus 100 may very accurately position the moving body 110, because the apparatus has a mechanism that moves the moving body 110 without it contacting the fixed body 120, such that vibrations hardly occur in the apparatus.
Recently, a more accurate positioning, for example, in the order of 10 nm or less, is required, when an ultrahigh-accurate process or an ultrahigh-accurate measurement is performed by using a static-pressure bearing apparatus. So, minor vibrations of a moving body have come to be a problem, though such minor vibrations were not regarded as being a problem previously.
Vibrations of a moving body occur due to unstable pressurized gas flowing through the bearing clearance G1. The flow of pressurized gas is stable in a laminar region and a turbulent region. In contrast, the pressure of pressurized gas fluctuates and the flow of pressurized gas is unstable in an intermediate region. The transient region between a laminar region and a turbulent region has a Reynolds number Re of 2,000 to 3,000. So, the transient region may be changed into a laminar status, if the Reynolds number of the transient region is changed to be not more than 2,000. Vibrations do not occur in the laminar state, because the pressurized gas flows smoothly.
To suppress vibrations of a moving body in a static-pressure bearing apparatus, for example, Patent document 1 discloses a method for improving patterns and shapes of passages in a vent groove to provide a passage configured to suppress the occurrence of turbulent flows. Patent document 2 discloses a method for reducing the viscous resistance of the flow of gas and preventing the occurrence of a turbulent flow, by reducing the surface roughness of a sliding surface of a static-pressure pad. Patent document 3 discloses a method for preventing vibrations by roughening gas outlets of a sliding surface of a static-pressure pad to eliminate the transition region between a turbulent flow and a laminar flow and for generating a stable turbulent region instead of the transition region. The approach in Patent document 3 is opposite to the approach in Patent documents 1 and 2.
Patent document 1: JP2003-194059
Patent document 2: JP6-307449
Patent document 3: JP3260869